"Until now all scenarios are possible. So please, it is very important that we do not talk and say there is a specific scenario," Sisi said, according to Reuters. "This could take a long time."

Sisi said that underwater equipment from Egypt's offshore oil industry was being brought in to help the search.

"They have a submarine that can reach 3,000 meters (nearly 10,000 feet) under water," he said in the televised speech. "It moved today in the direction of the plane crash site because we are working hard to salvage the black boxes."

An oil ministry source said Sisi was referring to a robot submarine used mostly to maintain offshore oil rigs. It was not clear whether the vessel would be able to help locate the black boxes, or would be used in later stages of the operation.

Air crash investigation experts say the search teams have around 30 days to listen for pings sent out once every second from beacons attached to the two black boxes. At this stage of the search they would typically use acoustic hydrophones, bringing in more advanced robots later to scan the seabed and retrieve any objects once they have been found.

Sisi's remarks came as an audio recording of the EgyptAir pilot and air-traffic controllers emerged that suggested nothing was amiss 2½ hours before the jet disappeared from radar screens over the Mediterranean Sea.

“Hello, hello, EgyptAir 804, flight level 370, squawk number 7624,” the pilot of EgyptAir MS 804 is heard telling Zurich Airport in Switzerland in audio recorded by the website liveatc.net at around 11:51 p.m. Wednesday local time.

"EgyptAir 804 radar contact," an air-traffic controller says, according to the audio. The pilot replies, "Thank you so much."

The plane was traveling from Paris to Cairo when it disappeared with 66 people on board shortly after leaving Greek airspace at around 2:30 a.m. Thursday Cairo time (8:30 p.m. Wednesday ET).